Waveguide junction



195.6 H. R. BARKER, JR 2,766,431

WAVEGUIDE JUNCTION Filed Feb. 8, 1952 +t- --l+1 B F 4 INVENTOR HARRYR. BARKER JR.

'ATTORN EY United States Patent Ofiice 2,766,431 Fatented Oct. 9, 1956 WAVEGUIDE JUNCTION Harry R. Barker, J12, Boston, Mass, assignor to Sylvauia Electric Products, Inc., a corporation of Massachusetts Application February 8, 1952, Serial No. 270,663

' 4 Claims. (Cl. 333-9 The present invention relates to couplings between waveguides and more particularly to directional couplers and to single-hole directional couplers.

The problem of coupling energy from one waveguide to another is attendant with the ditnculty that because of the discontinuity represented by the coupling path, multiple reflections are produced. The limitations on the control of propagation and, to some extent, the loss of signal energy, represent a source of difliculty in waveguide systems using such couplings. This is particularly true in directional couplers whose purpose is to couple energy from a main waveguide where energy may travel in both forward and backward directions to an auxiliary waveguide where forward energy of the main guide is to be coupled in only one direction in the auxiliary guide; and in this direction in the auxiliary waveguide there should be an absolute minimum of coupling from the backward wave in the main guide.

Directional couplers are not necessarily required to couple more than a small fraction of the total primary energyinthe main waveguide to the primary branch of the auxiliary waveguide. However, the directional coupler is expectedto suppress backward wave energy in the forward energy branch of the auxiliary guide.

An object of the present invention is to reduce the eflect of discontinuities in a coupling between waveguides having a common wall. More particularly an object of the invention is to reduce the effects of discontinuities in hole couplers.

In another aspect, an object of the invention is to improve therratio of forward wave coupling into one branch of the auxiliary waveguide of a directional coupler to the backward wave coupling into that auxiliary waveguide branch.

Further objects and features of novelty will be better understood and more fully appreciated from the following detailed disclosure of an illustrative embodiment which is shown in the accompanying drawings. In these drawings,

Figure 1 is a lateral view, partly in cross-section, of a hole directional coupler;

Figure 2 is a view of the coupler of Figure 1, looking upward, with portions broken away for clarity;

Figure 3 is a line drawing corresponding to Figure 1 illustrating one set of propagation paths; and a Figure 4 is a line drawing similar to Figure 3 illustrating another set of propagation paths.

In Figure 1, two lengths of rectangular waveguide are shown, the main waveguide having a common wall 12 with an auxiliary waveguide 14 with the axes of the waveguides at an angle of 35. These waveguides operate in the TEormode. Accordingly, each waveguide of rectangular cross-section has an opposed pair of walls whose width a is greater than the width b of the other pair of opposed walls. Common wall 12 of these two 7 waveguides is formed of portions of the wide walls a of the main and auxiliary waveguides.

'In the coupler illustrated an aperture 16 is formed as a stepped hole in common wall 12 for coupling energy from main waveguide 10 into auxiliary waveguide 14. The top wall of the main waveguide is shown joined to the bottom waveguide wall of the auxiliary waveguide. Because of the thickness of these two walls some advantage is realized by making a larger hole in the lower Waveguide wall of auxiliary waveguide 14 than the hole in the top wall of main waveguide 10. The particular form of this coupling aperture influences the directivity and the coupling. In each of the Figures, energy can enter waveguides it) from either the left or the right-hand end and in broad concept some energy from either direction of propagation in the main waveguide can go in either direction in the auxiliary waveguide. Thus in Figure 3 the forward signal path is represented by an input A to the main waveguide, most of which emerges as output from the other end of main waveguide 10 as signal A0. Some small portion of this will penetrate aperture 16 and divide into signal paths A1 and A2. The energy that penetrates the coupling aperture divides, and is propagated in both branches of the auxiliary waveguide. Similarly, another propagation path can exist as illustrated by Figure 4 in which a backward signal B enters from the right-hand end of the main waveguide 1t) and emerges, in large measure, as backward wave output Bo at the left-hand end of the main waveguide. A sample of this energy appears in auxiliary waveguide and is transmitted in two directions, B1 and B2.

Where the hole coupler is used for directional propagation for sampling the forward energy in the main waveguide substantially to the exclusion of any backward energy, a conventional matching load or absorber is disposed in one branch of the auxiliary waveguide. In Figures 1 and 2 this matching load is formed in the right-hand branch of auxiliary guide 14 and takes the form of a laminated absorber 18 including a thin, flat, central carbonized card 18a, and a pair of dielectric plates 16]) whose purpose is to elfectively shorten the length of waveguide occupied by the absorber. This load has a slant surface of half-wavelength taper for minimizing the discontinuity and increasing its broad band eifectiveness.

Referring to Figures 3 and 4 the portion of transmissions A and B represented by B1 and by A2 are effectively suppressed by absorption in the righthand branch of the auxiliary waveguide. Appearing in the left-hand branch of the waveguide are samples A1 and B2 of the propagations in the main waveguide. When the angle between the two waveguides is proper, excellent discrimination in the relative strength of A1 and B2 can be realized.

Characteristics of directional couplers of great interest are the coupling factor and the directivity.

The first of these is defined by the ratio of A1 to A, or B1 to B. A1 is the desired output sample in the auxiliary waveguide and A is the input to the main waveguide. The directivity is the ratio between the desired sample, A1, to the spurious transmission B2 from the main waveguide. Basicaliy this spurious sample B2 is of a far lower order of magnitude than A1, as a characteristic of the directional coupler, even presuming equal signal input at both ends of the main waveguide.

- in practice the load that is coupled to one branch of the auxiliary waveguide is not always a perfect match and accordingly there is some reflection of signal B1 of the backward transmission in the main waveguide into the branch of the auxiliary waveguide receiving the sample A1 of the forward signal. Also, the inherent sample B2 of the reverse transmission may not be disregarded. These two quantities add to reduce the directivity of the directional coupler. By mounting a wedge 20 opposite hole 16 a vast improvement has been realized in respect to this directivity Without, however, reducing the coupling from the main waveguide in the forward direction to the arm of the auxiliary waveguide that is to receive the sample of the forward propagation. This wedge is in this embodiment formed of a 150 apex having a core of wood and a high-absorption surface layer, here of polyiron. The slant faces of this wedge are made one quarter guided wavelength measured along the slant side at the mid-band frequency for which the waveguide system is designed, and the height of the wedge is approximately b/3. Ey means of this slant-sided element 20 considerable improvement has been attained, to the extent that, whereas a 25 db separation between signals A1 and B: has previously been realized, a 50 db difierence between these signals has now been attained without any decrease of coupling. The coupling factor A1 to A has been 25 db down, and in the new construction it is still 25 db. The coupling factor has been of the same order of magnitude as the directivity heretofore, and the directivity has now been vastly improved without loss of coupling. Element 20 serves to modify the cross-section of the waveguide paths opposite coupling hole 16; and it is also a matching element in that the impedance match looking in at the left-hand end of the auxiliary guide is improved.

The branch of the auxiliary waveguide into which signal sample A1 is propagated is usually connected to a matched load, not illustrated, that commonly is a crystal detector and a probe. The matched load represented by termination 18 may manifestly be omitted where there is a useful purpose for reverse signal sample B1.

Various modifications and applications of the features in the foregoing coupler will naturally occur to those skilled in the art and therefore the appended claims should be accorded a broad interpretation consistent with the spirit and scope of the invention.

What I claim is:

1. A pair of rectangular waveguides having a broad wall of one against a broad wall of the other and angled in relation to each other and embodying a coupling aperture through said walls providing a transmission passage from one waveguide to the other, and an impedance matching element extending from the side of one of said waveguides at a point directly opposite the coupling aperture, the sides of which matching element have substantial slope from an apex directly opposite the aperture, the sloping sides extending in opposite directions.

2. A pair of skew rectangular waveguides having a broad wall of one against a broad wail of the other and both the walls apertured to provide a coupling passage between the waveguides, and an impedance matching element extending from the side of one of said waveguides directly opposite the coupling aperture having slant faces from the base of the element to constitute progressively reduced cross-sections toward the side of the waveguide in which the coupling aperture is formed the faces having substantial slope from an apex at a point directly opposite the coupling paxage and extending along the waveguide which contains the matching element.

3. A directional coupler including a pair or new rectangular waveguides having a broad wall of one juxtaposed against a broad wall of the other and both the juxtaposed broad walls having a coupling hole therein, and a broad-based wedge on the side of one waveguide opposite the coupling hole and having an apex opposite the coupling hole, said wedge having exposed surfaces of high energy absorption characteristics, the apex angle of the wedge being obtuse, and the sides of the wedge sloping relative to the length of the waveguide in which the wedge is disposed.

4. A pair'of skew waveguides having a wall common to both and a coupling aperture extending through the common wall and providing a transmission passage between the waveguides, said waveguides including an input waveguide and an output waveguide, and an impedance matching element extending from the side of the output waveguide opposite the coupling aperture and having an apex directly opposite that aperture, the sides of which element have substantial slope away from said apex along said output waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,453,760 Schelle'ng Nov. 16, 1943 2,478,317 Purcell Aug. 9, 1949 2,562,281 Mumford July 31, 1951 2,605,400 McClain July 29, 1952 2,609,450 Early Sept. 2, 1952 2,615,982 Zaslavsky Oct. 28, 1952 2,618,744 Braden Nov. 18, 1952 2,643,295 Lippmann June 23, 1953 

